3. PURPOSE AND FUNCTION OF SPILLWAYS

3.1. FLOOD CONTROL

The spillway must be able to pass the design flood without endangering the dam and without generating greater downstream danger than would occur without the dam. Gated and ungated spillways do this in different manners and with different degrees of reliability.

3.2. FUNCTION OF UNGATED SPILLWAYS

The ungated spillway passes flow in accordance with the elevation of the reservoir water surface above the spillway crest (the spillway head). In some ways the ungated spillway provides a disadvantage in that it provides little or no control over the rate at which water is released to the river downstream. The hydrograph of the inflowing flood, and the initial contents of the reservoir, govern the rate at which water is released to the river downstream of the dam. If, at the start of a flood, a significant volume is available in the reservoir for temporary flood storage, the peak rate of flow released downstream will be much less than that of the incoming flood. However, if little volume is available for temporary flood storage at the beginning of a flood, the peak inflow rate may not be significantly reduced below the peak of the incoming hydrograph. The process of proportioning the capacity of an ungated spillway involves routing the computed design flood hydrograph through the reservoir and assessing the maximum reservoir water-surface elevation that occurs. In general the starting reservoir water-surface elevation for the routing process should be at the crest of the spillway in order to allow for the possibility of an antecedent flood. The top of dam elevation is then set above the maximum water-surface elevation by the required amount of freeboard for safety. The required freeboard will depend upon the design of the dam and the need to be conservative. In general freeboard should never be less than one meter, and for embankment dams should be in the range of 3 to 4 meters. If the spillway is being proportioned to safely pass the probable maximum flood, there is less need to be conservative in setting the minimum required freeboard.

Once the capacity of the ungated spillway has been set, the designer tacitly assumes that the spillway will be capable of safely passing all future extreme floods. Such a project has little need for a real-time forecasting system to forecast inflows to the dam. The exceptions are dams where programmed flood storage in the reservoir is variable depending upon season and conditions in the upstream watershed. Operating rule curves are typically developed for large storage dams in California where snow pack in the Sierra Nevada Mountains is significant and does not begin to melt until spring. In those cases the volume of storage reserved for flood control will depend upon the date and the total forecasted runoff (including snowmelt). A description of such rule curves and their development has been provided in a General Report in the proceedings of the International Symposium on Dams and Extreme Floods [7]. For those projects, water is released as fast as allowable any time that the reservoir contents exceed that given by the established rule curve.

3.3. FUNCTION OF GATED SPILLWAYS

3.3.1. Flexibility of operation

Gated spillways provide a flexibility of operation that is not provided by ungated spillways. For a gated spillway it is possible to incorporate strategy in passing extreme floods (R.5, R.17). If the area downstream of the dam is inhabited it will be desirable to limit downstream releases to that flow rate at which flooding along the river would begin. That maximum desirable rate of downstream release can be used in development of the operation strategy for the spillway. When an inflow flood begins, the decision can be made to release either the incoming flow rate or the maximum desirable downstream rate, whichever is less. If the release is less than the inflow rate, the reservoir contents will increase. If the release rate is more than the flood inflow rate, the contents of the reservoir will decrease. Basically such simple operational policy can be followed until the gates are fully open. No further changes can be made until the reservoir begins to fall again and the operator can begin to close gates again. If stream gages which measure and communicate the inflow rate to the dam operator are not available, the inflow rate can be judged by the rate of rise or fall of the reservoir surface provided the reservoir surface elevation is being measured, the reservoir surface area is known as a function of elevation, and the outflow rate from the dam are known. Following such simple operational rules will insure that the available storage space in the reservoir is used to the extent possible while preventing unacceptable flooding. R. Pike and G. Grant (South Africa) describe the development of simple operational rules for operation of gate spillways and clearly state the benefits of their simplicity (R.17) regarding dam safety during passage of extreme floods.

3.3.2. Operating rules

The operation of gated spillways can benefit from the development of sophisticated operating programs. The successful development and use of such programs require dependable data acquisition programs which, as a minimum, measure and transmit real-time data on precipitation in the drainage catchment and flow rate in the river upstream. If real-time forecasts of future inflow rates are to be made, other data such as temperature, humidity, and snow pack much also be taken, recorded and transmitted to the dam control center. The result is a program which provides real time information to the dam operator which, if everything is working properly, make it easy for the operator to make safe decisions on the operation of the projects spillway gates. Such programs have been developed and installed in South Africa (R. 17) and are in the process of being developed and installed in Spain (R.25, R.5). Morocco has also implemented some systems to provide real-time forecasting (R.4).

In the extreme, such programs can be automated to the extent that the program will operate the spillway gates based on existing and predicted flood inflow and current contents of the reservoir. However, the equipment required to obtain and transmit data to the dam control center requires significant maintenance, inspection, and testing and is most likely to cease operating during severe storms when the data is most needed. In addition, operation of spillway gates cannot be guaranteed during times of extreme floods. Diligent regular maintenance is required to assure the operation of gates when needed. Q. Shaw and W. Haken of France point out that the reliability of gates varies with the level of development of the country (R.18). Particularly in countries that are and have been subject to war, the infrastructure may have deteriorated to the point where the reliability of gate spillways is very uncertain.

R. Pyke and G. Grant point out that pitfalls sometimes arise when sophisticated (black-box) programs for dam operation are used (R.17). In South Africa, where such black box programs have been use for some time, a backup system for operation has been found to be very important. The operator is often totally unaware of how the black-box program makes decisions on operation because the algorithm for operation has been developed others. As a result the operator has no means for confirming that the black-box program is performing as intended. If the black box system fails, and no back-up strategy is in place, the operator has no means for making decisions on gate operation. As a back up, South Africa has developed simple operating rules which the operator can follow as necessary in making decisions as to if and when to open spillway gates. Pyke and Grant point out that black-box operating systems, are easier for everyone to understand if the algorithm the program uses is based on the same simple rules. If the operator can understand the simple system, he has the means to check on the operation of the black box program, and ascertain that the program is operating correctly (R.17)

4. HYDROMECHANICAL EQUIPMENT FOR GATED SPILLWAYS

4.1. TYPES OF GATES AND THEIR DESIGN

A wide variety of spillway gates have been developed and used through the history of dam design and construction. The most common mechanical types are (R.10, R19):

4.1.1. Vertical-lift gates

These gates generally are wheel gates (except for very small dams when they may be slide gates). For surface spillways on large dams they require a substantial overhead structure and usually a gantry crane for their operation and removal for maintenance. Maximum sizes range to 20m by 20 m. Provided the gate lip is properly designed, they are reliable for closure under full flow. They require a bar seal across the bottom and music-note seals on the sides. Gate slots are required and flow is disturbed by them which makes them somewhat less efficient in passing flow than a radial gate. When vertical-lift gates are used on tunnel spillways a bar seal is required on the bottom and music-note seals are required on the top and edges. Two music-note seals are required on the top of the gate in order to prevent severe vibration caused by cavitation from leakage at the top of conduit when the gate is opened (R.1)

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4.1.2. Radial gates

Radial gates are the most popular because they are in general cheaper to construct, their lifting load is smaller than vertical-lift gates, and they are structurally strong. For surface spillways maximum size ranges to 40m wide by 10m high and 20m by 20 m. Radial gates have better discharge characteristics than vertical lift gates because they do not require gate slots which disturb flow. Stiff bar seals are required across the bottom and music-note rubbing seals along the vertical sides. Top sealing radial gates are frequently used for flow control in tunnel spillways (R.1, R.39). In general it is not a good practice to allow flow to overtop large radial gates.
If large flows pass over the gate, serious vibrations can occur which may structurally damage the gate (R.10, R.34, R.37). Where a significant amount of debris accumulates upstream of the gates, a movable flap is sometimes provided at the top of the radial gate in order to provide a capability for passing trash (R.6, R.34). B. Sagar of the USA recommends that the top of radial gates be fitted with a shaped overflow crest, because the hinges of these flaps require continual maintenance to insure that they will operated when needed (R.34).

A radial gate must be structurally able to resist any forces tending to rack the gate when it is being raised or lowered. In some cases side wheels are provided on each side of the gate in order to minimize the possibility of the gate becoming jammed (R.36). In July 1994 the rod of a hydraulic cylinder failed while one of the radial gates on Itaipu dam was being lowered. Bulkheads had been installed upstream of the gate to allow for maintenance of the gate in the dry. Detailed investigations show that racking of the gate caused the gate to vibrate in turn causing the loads on the hydraulic cylinders to increase to the point where one rod (in weakened condition due to corrosion) fractured (R.43).

4.1.3. Flap gates

Flap gates are used on surface spillways when close control of a reservoir water surface is required and the maximum operating head will be relatively small. Maximum sizes range to 70m wide but because they incur heavy lifting loads they have been limited in height to approximately 3m. Seals are continuous across the bottom of the gate with rubbing seals on each side. They are subject to severe vibrations unless splitters are installed along the crest of the gate to provide aeration to the cavity below the nappe (R.20, R.34, R4O). Flap gates are excellent for use where a great deal of trash accumulates in the reservoir since they can readily pass trash over the gate. In Mozambique studies are underway to remove and replace 38 flap gates with radial gates in order to allow increased reservoir storage (R.10).

4.1.4. Rubber dams

These inflatable dams are actually used as a gate primarily to control the upstream water surface. Maximum sizes range to 100m long by 3m high. They do not control discharge as well as a flap gate because flow concentrates at the center of the bag instead of distributing evenly across the dam. It is impossible to develop an accurate rating for these rubber dams. They have the advantage that they are economical to install in cases where it is desirable to increase storage in the reservoir without spillway capacity.

4.1.5. Segment gates

Segment gates are composed of two independently-operated gate leafs. Generally the leafs are essentially wheeled vertical-hoist gates. The gates provide much operational flexibility and are generally used where long spans must be covered. Maximum sizes range to 70m in length. The leafs and the hoisting system are designed for a variety of operational conditions:
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-Flow over the top leaf only
-Flow under the bottom leaf only
-Flow over the top leaf and flow under the bottom leaf


They are ideal where large flows must be controlled and where a good deal of floating debris must be handled. For this reason they are more commonly used on dams controlling flow in a run-of-the-river fashion. M. Bubenik states that 43 such gates are in use in the Czech Republic (R.20). Nine of these gates are in use in Guatemala (R. 10).

4.2. HOISTING EQUIPMENT

In general two types of hoisting equipment are available for operating gates: cable hoists and hydraulic cylinders. The choice between these two is generally a matter of choice. There are certain specific situations for which only one type can be used. For example, only hydraulic cylinders can be used to operate a top-sealing radial gate operating in a tunnel spillway. Often a second hydraulic cylinder and a cam are included to force tight sealing of the gate when closed. The operating cam allows the gate to be pulled slightly away from its sealed position; the gate can then be moved without damaging the seals. Similarly, operating requirements of a vertical-hoist gate, operating at the bottom of a tall control shaft, are best met by a cable hoist. In the latter case a hydraulic cylinder could be used, but it would require a very long hoisting rod and special equipment to lift the gate completely out of the tower for maintenance, whereas a cable hoist can be arranged to lift the gate completely out of the tower without special equipment.

Slide gates are sometimes raised and lowered by means of a motor driven rotating screw stem. However, such cases are limited to relatively small gates.
In some cases, a gantry crane, or mobile crane, is provided to raise and lower vertical-lift gates. ln such cases the gantry crane is also required to handle other equipment su ch as trashracks and bulkheads. Since the crane can only handle one gate at a time, its use is inconvenient and the process may be too slow at times when extreme floods need to be passed and many gates must be controlled (R.26).

Having only one hoist for several gates introduces additional questions of reliability since if the single hoist fails, none of the gates can be operated.

4.2.1. Gable hoists

In general, cable hoists may be used for capacities up to 50 tons (R.19). If well maintained they provide reliable service with a long service life. Cable hoists have been widely used throughout the world and are manufactured in most developed countries. Thus, in many cases it may be easier to obtain replacement parts for cable hoists than for equivalent hydraulic systems. Gates controlled with cable hoists must be designed to close under their own weight since the cable hoist can provide only lifting force. Vertical-lift gates, suspended in tall towers, have experienced serious vibration problems when the gate was lowered into high-velocity flow due to the elasticity of a long cable [5]. A brake must be incorporated on the hoist in order to control the speed at which the gate is lowered. When used on radial gates, a cable is required on each side of the gate in order to avoid any tendency of
the gate to rack. For large radial gates, two cable drums (one for each side of the gate) may be operated by independent electrical motors or by a single motor with a shaft connecting the two drums. If two electrical motors are used, electrical synchronizing equipment is required to maintain the same lifting force and speed on both sides of the gate. Lifting cables are usually fastened to the bottom of the upstream side of the gate since this arrangement provides for the maximum leverage and the smallest cable force required to lift the gates. Either ordinary steel or stainless wire rope can be used on the cable drums. Since the cable hoists are generally mounted on a platform above the gates and are not sheltered, the cables are subject to the weather and consequent corrosion (R.26, R.34).

4.2.2. Hydraulic cylinders

Hydraulic cylinders are now used widely throughout the world for operation of all types of gates. They can provide a larger lifting force than cable hoists and if fabricated as double-acting cylinders and can provide both lifting and pushing forces. Hydraulic cylinders are preferred for gates with potential for vibrations since the action of the hydraulic cylinder serves as an additional damping force on the gate [5]. In Portugal most cable hoists have been replaced by hydraulic cylinders in order to make maintenance easier (R.12).

4.3. AUTOMATIC GATES

In attempts to improve reliability and dam safety, a number of schemes have been developed for automating the operation of gates. In most cases the automatic features have been provided by a system of floats and counterweights. In those cases it is intended that the spillway gate automatically open if the reservoir water surface rises. Although such automatic operation is theoretically interesting, the float and counterweight systems have in general proven to be unreliable and to require stringent maintenance. There are 15 projects in Portugal in which automatic gates are incorporated. The largest of the gates is a 27m wide by 5.3m high radial gate. In 1987 a cable broke on a float for an automatic gate at Moule Nova Dam in Portugal (R.12). As a result the gates were overtopped. Recently introduced dam­safety regulations require that all automatic gates in Portugal be replaced by gates having positive control (R.12).

Other types of automatic gates include erodible fuse plugs which have often been used particularly for emergency spillways. Systematic studies of the design of fuse plugs have been carried out by many entities [9]. However, their use is declining because of a concern that the fuse-plug embankment may not erode as planned.

The most recent development in automatic gates is the Hydroplus Fuse Gate which is constructed of steel and placed on a modified spillway crest. Inidividual fuse gates fail at predetermined reservoir elevations in order to prevent overtopping of a dam during passage of an extreme flood and to avoid sudden large spills. These fuse gates provide an economical means of increasing reservoir storage while maintaining the maximum spillway capacity (R.28). New developments in fuse-gate design are described for a spillway modification at Montsalvens Dam (R.16). At Montsalvens Dam the fuse gates are mounted in a narrow channel where the velocity of approach would be increased significantly by the tipping of one gate. The increased velocity of approach would lower the water surface in the channel below that of the reservoir. To counteract this effect, the fuse gates were designed with piping connections to the main reservoir (R.16).

A special case of gate automation is discussed in (R.15) for the Pueblo Viejo Dam in Guatemala where an increase in reservoir storage was desired. In this case ungated spillway crests were modified and radial gates were installed. The existing spillway crest was lowered in order to provide sufficient capacity to pass the 10,000­year flood (the original design flood had been the 1,000-year). The radial gates were controlled with hydraulic cylinders and were counterweighted in order that the gate would open automatically in case the hydraulic operation failed (R.15). For this particular project a number of redundancies in the hydraulic power system were incorporated in order to insure the reliability of the hydraulic system. The counter weight was a fail-safe measure.

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